Filter

Abstract

A filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a washcoat comprising a plurality of solid particles wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, and wherein the second mean pore size is less than the first mean pore size.

1. A filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a washcoat comprising a plurality of solid particles wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, and wherein the second mean pore size is less than the first mean pore size. 
     
     
       2. A filter according to  claim 1 , wherein a first mean pore size of the porous structure of the porous substrate is from 8 to 45 μm. 
     
     
       3. A filter according to  claim 1 , wherein the washcoat loading is >0.50 g in −3 . 
     
     
       4. A filter according to  claim 3 , wherein the washcoat loading is >1.00 g in −3 . 
     
     
       5. A filter according to  claim 1 , comprising a surface washcoat, wherein a washcoat layer substantially covers surface pores of the porous structure and the pores of the washcoated porous substrate are defined in part by spaces between the particles (interparticle pores) in the washcoat layer. 
     
     
       6. A filter according to  claim 5 , wherein a mean interparticle pore size of the washcoat layer is from 5.0 nm to 5.0 μm. 
     
     
       7. A filter according to  claim 1 , wherein a mean size of solid washcoat particles is greater than the first mean pore size. 
     
     
       8. A filter according to  claim 7 , wherein the mean size of the solid washcoat particles is in the range 1 to 40 μm. 
     
     
       9. A filter according to  claim 1 , wherein the pores at a surface of the porous structure comprise a pore opening and the washcoat causes a narrowing of substantially all the surface pore openings. 
     
     
       10. A filter according to  claim 1 , wherein the washcoat sits substantially within the porous structure of the porous substrate. 
     
     
       11. A filter according to  claim 9 , wherein a mean size of solid washcoat particles is less than a mean pore size of the porous substrate. 
     
     
       12. A filter according to  claim 11 , wherein a mean size of solid washcoat particles is in the range 0.1 to 20 μm. 
     
     
       13. A filter according to  claim 11 , wherein a D90 of solid washcoat particles is in the range 0.1 to 20 μm. 
     
     
       14. A filter according to  claim 1 , wherein the washcoat is coated on inlet surfaces, outlet surfaces or both the inlet and the outlet surfaces. 
     
     
       15. A filter according to  claim 14 , wherein both the inlet and the outlet surfaces are washcoated and wherein a mean pore size of washcoat on the inlet surfaces is different from a mean pore size of washcoat on the outlet surfaces. 
     
     
       16. A filter according to  claim 15 , wherein the mean pore size of washcoat on the inlet surfaces is less than the mean pore size of washcoat on the outlet surfaces. 
     
     
       17. A filter according to  claim 16 , wherein a mean pore size of washcoat on the outlet surfaces is greater than a mean pore size of the porous substrate. 
     
     
       18. A filter according to  claim 1 , wherein the porous substrate is selected from the group consisting of a ceramic wallflow filter, a metal filter and a ceramic foam. 
     
     
       19. A filter according to  claim 18 , wherein the metal filter is selected from the group consisting of a sintered metal filter, a partial filter and combinations thereof and optionally comprises a wire mesh. 
     
     
       20. A filter according to  claim 1 , wherein the washcoat is a catalytic washcoat. 
     
     
       21. A filter according to  claim 20 , wherein the catalytic washcoat is selected from the group consisting of a hydrocarbon trap, a three-way catalyst, a NO x  absorber, an oxidation catalyst, a selective catalytic reduction (SCR) catalyst and a lean NO x  catalyst. 
     
     
       22. A filter according to  claim 20 , wherein the catalytic washcoat comprises at least one molecular sieve. 
     
     
       23. A filter according to  claim 22 , wherein the at least one molecular sieve is a small, medium or large pore molecular sieve. 
     
     
       24. A filter according to  claim 22 , wherein the at least one molecular sieve is selected from the group consisting of AEI, ZSM-5, ZSM-20, ERI, LEV, mordenite, BEA, Y, CHA, MCM-22 and EU-1. 
     
     
       25. A filter according to  claim 22 , wherein the molecular sieve is un-metallised or is metallised with at least one metal selected from the group consisting of groups IB, IIB, IIIA, IIIB, VB, VIB, VIIB and VIII of the periodic table. 
     
     
       26. A filter according to  claim 25 , wherein the molecular sieve is metallised and the metal is selected from the group consisting of Cr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga and the precious metals Ag, Au, Pt, Pd and Rh. 
     
     
       27. A filter according to  claim 26 , wherein the metal is selected from the group consisting of Cu, Pt, Mn, Fe, Co, Ni, Zn, Ag, Ce and Ga. 
     
     
       28. A filter according to  claim 27 , wherein the metal is selected from the group consisting of Ce, Fe and Cu. 
     
     
       29. An exhaust system for a positive ignition engine, which system comprising a filter according to  claim 1 . 
     
     
       30. An exhaust system according to  claim 29 , comprising means for injecting a reductant fluid into exhaust gas upstream of the filter. 
     
     
       31. An exhaust system according to  claim 30 , wherein the reductant fluid is a nitrogenous compound. 
     
     
       32. A positive ignition engine comprising an exhaust system according to  claim 29 . 
     
     
       33. A positive ignition engine according to  claim 31 , fuelled with a hydrocarbon fuel selected from the group consisting of gasoline, gasoline blended with methanol and/or ethanol, liquid petroleum gas and compressed natural gas. 
     
     
       34. A method of trapping particulate matter (PM) from exhaust gas emitted from a positive ignition engine by depth filtration, which method comprising contacting exhaust gas containing the PM with a filter comprising a porous substrate having inlet and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a washcoat comprising a plurality of solid particles wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, and wherein the second mean pore size is less than the first mean pore size.